Steel plate subjected to heat treatment and process for producing the same

ABSTRACT

The present invention provides a steel sheet for heat treatment consisting essentially of, by mass %, 0.05-0.09% C, below 1% Si, 1.6-2.4% Mn, 0.02% P or less, 0.02% S or less, 0.01-0.1% sol. Al, 0.005% N or less, 0.0003-0.003% B, Ti in a range satisfying (48/32)S+(48/14)N≦Ti≦2[(48/32)S+(48/14)N], and the rest of Fe, wherein the average diameter of iron carbides precipitating in steel is 2 μm or smaller. The steel sheet for heat treatment according to the present invention has high strength and excellent hydrogen embrittlement resistance after being press formed and quenched, so that the steel sheet is well suited for automobile construction members.

TECHNICAL FIELD

The present invention relates to a steel sheet for heat treatment that can be steadily imparted with high strength and excellent hydrogen embrittlement resistance by heat treatment conducted after forming process such as press forming, and a manufacturing method thereof.

BACKGROUND ART

In view of weight reduction for fuel economy and safety for cabin passengers/crews against an accident and the like, high strength steel sheets are used in a car as body construction members, reinforcing members, and various other mechanical construction components. However, the use of high strength steel sheets causes various problems such as difficulty of forming intricately shaped components, and high frequency of occurrence of brittle fracture, that is, so-called hydrogen embrittlement (delayed fracture), which is caused by hydrogen-absorption into steel from an environment.

Generally, to meet the requirement of formability and high strength, such a method as described hereunder is employed. After being formed by, for example, press forming, a steel sheet such as a cold rolled steel sheet or a hot rolled steel sheet is heated by induction-heating method or furnace-heating method, and then subjected to quenching such as water quenching, oil quenching, or press quenching. Steel sheets of several types suitable for this method have been developed in the following prior arts.

According to Japanese Examined Patent Publication No. 3-2942, there is proposed a steel sheet for precise punching which is excellent in formability in various forming modes and quench-hardenability after short time and rapid heating. The steel sheet is made of a Cr and B-added steel containing 0.10-0.19% C and 0.7-1.5% Mn.

According to Japanese Patent No. 2713382, there is proposed a method for manufacturing a high strength automobile member excellent in hydrogen embrittlement resistance. The method is characterized in that a steel containing 0.2-0.5% C, 0.5-1.6% Mn and 0.5-1.5% Cr is treated with a lubricant film forming agent, then formed, and finally subjected to quenching and tempering treatment.

According to Japanese Examined Patent Publication No. 7-103420, there is proposed a method for manufacturing a member using a B-added steel. The method is characterized in that a B-added steel containing 0.15-0.40% C and 0.60-1.50% Mn is subjected to cold press forming, then heated at a quenching temperature of 850° C. to below 950° C., and water quenched at a quenching intensity of 0.35 cm⁻¹ to below 1.50cm⁻¹.

According to each of Japanese Unexamined Patent Application Publications No. 5-98356 and No. 5-98357, there is proposed a method for manufacturing a Ti and B-based high carbon steel sheet having excellent formability and toughness without tempering treatment. This method is characterized in that a (Ti) and B-added steel containing 0.15-0.40% C and 0.6-1.50% Mn is used to inhibit the precipitation of cementite. Concurrently, B is added to secure hardenability, and the precipitation of AlN (and TiN) is performed to inhibit abnormal growth of austenite grains.

According to Japanese Unexamined Patent Application Publication No. 6-116679, there is proposed a method for manufacturing a steel sheet and a safety component thereof against car collision, the safety component being fabricated by press quenching. The steel sheet is manufactured in such a manner that a Ti, Nb and B-added steel containing 0.20-0.40% C and 0.20-0.40% Mn is heated at a temperature of Ac₁ to (Ac₁+30)° C. for 1 to 20 hours after being hot rolled, and then cooled to a temperature below (Ac₁+30)° C. at a cooling rate of 20° C./s or less. The safety component is fabricated by press quenching in the following manner. After being formed into a predetermined shape, the steel sheet is then heated to 850° C. or above, cooled at a cooling rate of 80 to 150° C./s to a temperature range between 450 and 500° C. while being kept in a metal die, and further cooled at a cooling rate of 20 to 100° C./s to an ordinary temperature not exceeding 100° C. Consequently, the component has a tensile strength of 1150 N/mm².

According to Japanese Unexamined Patent Application Publication No. 8-269615, there is proposed a hot rolled steel sheet that can be imparted with wear resistance after being formed without impairing stretch flangeability. This is accomplished in such a manner that after being formed, the steel sheet is subjected to rapid heating such as induction quenching whereby to harden the surface without cracks. The hot rolled steel sheet consists of, by weight, 0.18-0.30% C, 0.01-1.0% Si, 0.2-1.5% Mn, 0.1-0.5% Cr, 0.0006-0.0040% B, 0.03% P or less, 0.02% S or less, 0.08% sol.Al or less, and 0.01% N or less, and balance Fe with inevitable impurities. The steel sheet has a mixed structure of ferrite and bainite.

According to Japanese Unexamined Patent Application Publication No. 10-96031, there is proposed a method for manufacturing a high carbon hot rolled steel sheet and a high carbon cold rolled steel sheet that are each excellent in ductility before being quenched and that are each capable of having a predetermined hardness and toughness after being quenched. According to the method, a Cr, Ti and B-added hot rolled steel sheet containing 0.25-0.65% C and 0.20-0.40% Mn is heated at a temperature of 650° C. to below Ac₁ for 10 to 30 hours, or is slowly cooled to (Ac₁−30)° C. at a cooling rate of 3 to 20° C./h or to (Ac₁−20)° C. at a cooling rate of 3 to 10° C./h after being heated at a temperature of Ac₁ to (Ac₁+30)° C. for 1 to 20 hours, followed, by necessary, by being cold rolled at a reduction rate of 30 to 70% and heated at a temperature of 650° C. to below Ac₁ for 20 seconds or more.

According to Japanese Unexamined Patent Application Publication No. 10-147816, there is proposed a method for manufacturing a high carbon steel sheet that is excellent in formability and that is capable of having sufficient strength after being formed and heat treated. The method is characterized in that a Cr, Ti and B-added steel containing 0.25-0.45% C and 0.2-0.5% Mn is hot rolled, coiled at a temperature between 550 and 600° C., pickled, heated in an atmosphere of 95vol.% or more of hydrogen at a temperature between Ac₁ and (Ac₁+30)° C. for 1 to 10 hours, slowly cooled to (Ar₁−50)° C. or less at a cooling rate of 3 to 20° C./h. Alternatively, the steel sheet is further cold rolled and annealed at a temperature of (Ac₁−10)° C. or less.

According to Japanese Unexamined Patent Application Publication No. 10-251757, there is proposed a method for manufacturing a high carbon steel sheet that is excellent in formability and that is capable of having sufficient strength through a post forming heat treatment. The method is characterized in that a Cr, Ti and B-added steel containing 0.25-0.45% C and 0.2-0.5% Mn is hot rolled at a finishing temperature between (Ar₃+20) and (Ar₃+50)° C., coiled at a temperature between 550 and 600° C., pickled, heated in an atmosphere of 95vol.% or more of hydrogen at a temperature between Ac₁ and (Ac₁+30)° C. for 1 to 10 hours, and slowly cooled to (Ar₁−50)° C. or less at a cooling rate of 3 to 20° C./h.

According to Japanese Unexamined Patent Application Publication No. 10-60522, there is proposed a steel sheet having excellent formability that can be imparted with sufficiently high strength through melting and rapid solidification using high density energy irradiation such as laser irradiation. The steel sheet is characterized by comprising 0.04-0.3% C and 3% Mn or less, and by receiving high density energy irradiation for a time that satisfies a predetermined formula, the bead pitch of high density energy irradiation being larger than 1 mm.

According to Japanese Unexamined Patent Application Publication No. 2000-144319, a steel sheet and a manufacturing method therefor are proposed, the steel sheet having sufficient formability adaptable for body construction members of car and high strength through post forming quenching. The steel sheet is made of a Ti and B-added steel containing 0.05-0.20% C, 0.8-2.0% Mn, and Ti in a range of 3.4×N(%) or less. The steel sheet is manufactured in such a manner that a steel slab having the aforementioned composition is hot rolled, and coiled at a coiling temperature of 600° C. or above. Alternatively, the hot rolled steel sheet is coiled at a coiling temperature of 480° C. or above, followed by being cold rolled and annealed.

However, the above prior arts have problems described hereunder.

In Patent Publication No. 3-2942, since the C content is high, hydrogen embrittlement resistance after quenching is not excellent. In addition, since the Mn content is as low as 0.7-1.5%, high strength after quenching can not be steadily obtained.

In Patent No. 2713382, since the C content is high, hydrogen embrittlement resistance after quenching is not excellent. Since the Mn content is as low as 0.5-1.6.%, high strength after quenching can not be steadily obtained. Further, since tempering is essentially required, manufacturing cost (heat treatment cost) is increased.

Similar problems arise in any of Japanese Examined Patent Publication No. 7-103420 and Japanese Unexamined Patent Application Publications No. 5-98356, No. 6-116679, No. 8-269615, No. 10-96031, No. 10-147816, and No. 10-251757. That is, since the C content is high, hydrogen embrittlement resistance is not sufficient, and since the Mn content is low, high strength after quenching can not be steadily obtained.

In Japanese Examined Patent Publication No. 10-60522, in the quenching by high density energy irradiation such as laser irradiation, heating is performed restrictively to a narrow linear range. As such, high strength is given to a limited portion, and thus high strength of the overall component can not be obtained. Further, since the bead pitch in the linearly heated portion is required to be larger than 1 mm, the entire surface cannot be uniformly quenched.

In Patent Publication No. 2000-144319, the C and Mn content ranges are so wide that high strength after quenching can not be steadily obtained. Excellent hydrogen embrittlement resistance can not be obtained either.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a steel sheet for heat treatment that can be steadily imparted with high strength and excellent hydrogen embrittlement resistance by heat treatment conducted after forming process such as press forming, and a manufacturing method thereof.

The object is achieved by providing a steel sheet for heat treatment consisting essentially of, by mass %, 0.05-0.09% C, 1% Si or less, 1.6-2.4% Mn, 0.02% P or less, 0.02% S or less, 0.01-0.1% sol.Al, 0.005% N or less, 0.0003-0.003% B, Ti satisfying formula (1), and the balance of Fe, wherein the average diameter of iron carbides precipitating in the steel is 2 μm or smaller. (48/32)S+(48/14)N≦Ti≦2[(48/32)S+(48/14)N]  (1)

In formula (1), each element symbol represents the content of each element, by mass %.

A steel sheet for heat treatment according to the present invention can be manufactured by a method comprising the steps of: hot rolling a steel slab having the aforementioned composition into a steel sheet; cooling the hot rolled steel sheet at an average cooling rate of 30° C./s or less; and coiling the cooled hot rolled steel sheet at a coiling temperature of 500° C. or above.

EMBODIMENTS OF THE INVENTION

The present inventors conducted study and research on a steel sheet that can be steadily imparted with high strength and excellent high hydrogen embrittlement resistance by heat treatment conducted after forming process such as press forming. Consequently, it is found that reduction in C content, addition of B, and control of iron carbides are effective. This will be described in more detail below.

1. Composition

C: C is an important element to enhance strength of steel sheet by heat treatment. C should be added in an amount of 0.05% or more to impart sufficiently high strength to steel sheet. On the other hand, however, when C content exceeds 0.09%, hydrogen embrittlement resistance after heat treatment is deteriorated. In view of these facts, C content is specified to 0.05-0.09%.

Si: Si can be appropriately added by necessity. However, Si content exceeding 1% not only deteriorates chemical conversion treatability, but also leads to an increase in manufacturing cost. In view of these facts, Si content is specified to 1% or less.

Mn: Mn is an essential element to steadily impart high strength independently of heat treatment conditions such as soaking temperature, holding time and cooling rate. Mn content less than 1.6% can not sufficiently stabilize hardenability of steel sheet; and on the other hand, Mn content exceeding 2.4% deteriorates press formability of steel sheet. For these reasons, Mn content is specified to 1.6-2.4%.

P: P is an impurity in steel. P content exceeding 0.02% deteriorates formability and weldability of steel sheet, so that P content is specified to 0.02% or less. Although P should be preferably removed as much as possible in steelmaking process, too much reduction of P content leads to an increase in manufacturing cost.

S: S is an impurity in steel. S content exceeding 0.02% deteriorates formability and weldability of steel sheet, so that S content is specified to 0.02% or less. Although S should be preferably removed as much as possible in steelmaking process, too much reduction of S content leads to an increase in manufacturing cost.

Sol.Al: Al is added as a deoxidizing agent and for precipitating N in the form of AlN. While sol.Al content less than 0.01% is not sufficiently effective, sol.Al content exceeding 0.1% saturates the effect, leading to an increase in manufacturing cost. For these reasons, sol.Al content is specified to 0.01-0.1%.

N: N is an impurity in steel. N content exceeding 0.005% deteriorates formability of steel sheet, so that N content is specified to 0.005% or less. Although N should be preferably removed as much as possible in steelmaking process, too much reduction of N content leads to an increase in manufacturing cost.

Ti: Ti combines with N in the form of TiN and thus prevents B from precipitating in the form of BN whereby enhancing the effect of B. Ti generates sulfides while a steel slab is being cooled after heating in advance of nitrides generation, so that, to completely delete solute N, Ti should be added in an amount greater than or equal to the atomic equivalents of N and S, that is, greater than or equal to (48/32)S+(48/14)N. On the other hand, however, when Ti is excessively added in an amount exceeding twice the atomic equivalents, TiC precipitates, so that the formability of steel sheet is deteriorated. For these reasons, Ti content is specified to the range of from (48/32)S+(48/14)N to 2[(48/32)S+(48/14)N]%.

B: B should exist in the form of solute B in steel so as to steadily obtain high strength independently of heat treatment conditions such as soaking temperature, holding time and cooling rate. B content less than 0.0003% does not sufficiently exhibit this effect. On the other hand, B content exceeding 0.003% not only saturates the effect of B, but also reduces productivity in steel sheet manufacturing process. For these reasons, B content is controlled to 0.0003 to 0.003%.

The strength enhancement of steel sheet can be more steadily implemented when at least one element selected from 0.1-2% Cr and 0.1-2% Mo is added in addition to the composition described above. The content of Cr and Mo is each specified to 0.1-2% for the reason that the content of 0.1% or less is insufficient to steadily implement the strength enhancement, whereas the content exceeding 2% deteriorates the formability of steel sheet.

While the rest is essentially Fe, small amounts of inevitable impurities and other elements may be included within a range that does not disturb the advantage of the present invention.

2. Iron Carbides

The average diameter of iron carbides precipitating in steel influences the dissolution of the iron carbides at heat treatment. The average diameter should be controlled to 2 μm or smaller so that the iron carbides can be dissolved into austenite in a very short time, leading to high strength after quenching.

3. Manufacturing Method

The steel sheet for heat treatment of the present invention can be manufactured by a method for manufacturing a steel sheet for heat treatment comprising the steps of: hot rolling a steel slab having the above-described composition into a steel sheet; cooling the hot rolled steel sheet at an average cooling rate of 30° C./s or less; and coiling the cooled steel sheet at a coiling temperature of 500° C. or higher.

In the above, the hot rolled steel sheet is cooled at the average cooling rate of 30° C./s or less for the reason that an average cooling rate exceeding 30° C./s generates second phases that deteriorate the formability of steel sheet. For the same reason, the coiling temperature is set to 500° C. or higher.

Alternatively, the steel sheet for heat treatment of the present invention can be manufactured by a method for manufacturing a steel sheet for heat treatment comprising the steps of: hot rolling a steel slab having the above-described composition into a steel sheet; cold rolling the hot rolled steel sheet; and annealing the cold rolled steel sheet for recrystallization, wherein the annealed steel sheet is cooled at an average cooling rate of 30° C./s or less to 400° C.

In the above, the annealed steel sheet is cooled at the average cooling rate of 30° C./s or less to 400° C. for the reason that generation of second phases is inhibited not to deteriorate the formability of steel sheet.

In the present invention, heating temperature of steel slab prior to hot rolling should be preferably controlled to 1200-1250° C. from the viewpoint of enhancing the formability. Finishing temperature at hot rolling should be preferably controlled to Ar₃−890° C. from the viewpoint of making the ferrite structure to be uniform and fine. When improving flatness of hot rolled steel sheet and deleting yield point elongation to enhance formability of hot rolled steel sheet, temper rolling should be preferably conducted at an elongation rate of 0.3-1.5% after coiling.

Even when manufacturing a cold rolled steel sheet, the above-described hot rolling conditions, that is, the slab heating temperature controlled to 1200-1250° C. and the finishing temperature controlled to Ar₃−890° C., should be preferably taken from the same viewpoint. Further, also the cooling rate after hot rolling should be preferably controlled to 30° C./s or less for a reason that when the average cooling rate from hot rolling final pass to coiling exceeds 30° C./s, second phases are generated whereby to reduce manufacturability.

Reduction rate at cold rolling should be preferably controlled to 60% or greater to obtain fine iron carbides having an average diameter of 2 μm or smaller, which are essential to the present invention. From the viewpoint of formability, annealing temperature should be preferably controlled to 670-720° C. at box annealing and to 690-730° C. or 800-850° C. at continuous annealing. When improving flatness of cold rolled steel sheet and deleting yield point elongation to enhance formability of cold rolled steel sheet, temper rolling should be preferably conducted at an elongation rate of 0.3-1.5% after annealing.

EXAMPLE 1

Slabs were cast after vacuum melting of steels 1-14 having composition shown in Table 1. After reheated at 1250° C., the slabs were each hot rolled at a finishing temperature of 870° C. into hot rolled steel sheets. The hot rolled steel sheets were each cold rolled to 1.2 mm, and subjected to 720° C.×2 min. annealing simulating continuous annealing. Thus produced cold rolled steel sheets 1-14 were cooled at an average cooling rate of 10° C./s, and temper rolled at an elongation rate of 1.5%. Further, the cold rolled steel sheets 13 and 14 were each heat treated at 600° C. to control the carbide diameter.

JIS No. 5 tensile test pieces were taken from the cold rolled steel sheets in the direction rectangular to the rolling direction (i.e., width direction) to measure mechanical properties.

Then, for these cold rolled steel sheets, tensile strengths were measured after quenching performed under the following three conditions:

Condition 1: Water quenching after 1000° C.×5 min. heating

Condition 2: Water quenching after 1000° C.×5 min. heating and air cooling to 800° C.

Condition 3: Water quenching after 900° C.×5 sec. heating

Condition 1 is an ideal solution treatment and quenching condition. Condition 2 is a condition for delayed quenching after solution treatment. Condition 3 is a condition simulating low temperature and short time solution treatment such as quenching after induction-heating. For a steel sheet for heat treatment according to the present invention, it is preferable that high strength can be steadily obtained after quenching under any of the conditions 1 to 3.

Further, 30×100 mm rectangular test pieces were cut out from the cold rolled steel sheets quenched under the condition 1, and bent to 180° at a radius of 10 mmR, being U-shaped. Then, the U-shaped test pieces were tightened with bolts at both ends of the test piece by a force corresponding to spring back, and immersed into a 0.1 N hydrochloric acid solution to measure time until cracking occurs. In this manner, the hydrogen embrittlement resistance was investigated. A criterion for excellent hydrogen embrittlement resistance is no crack occurring in at least 30 days (delayed fracture time: at least 30 days).

Table 2 shows mechanical properties, tensile strengths after quenching, and delayed fracture time.

Any of steel sheets 2, 7, and 11 to 13 has a high ductility (El) leading to excellent formability, a tensile strength of 1200 MPa or higher after quenching independently of the quenching conditions, and 30 days or longer delayed fracture time leading to excellent hydrogen embrittlement resistance.

In comparison, steel sheet 1 of comparative example has C content less than the present invention range, so that the tensile strength after quenching is insufficient. Steel sheet 3 has C content greater than the invention range, so that the delayed fracture time is as short as three days resulting in poor hydrogen embrittlement resistance. In addition, the sheet 3 has an average carbide diameter exceeding 2 μm, so that the tensile strength after quenching is insufficient at low temperature and short time solution treatment under the condition 3. Steel sheet 4 has Mn content less than the invention range, so that the tensile strength after quenching is insufficient under the condition 2. Steel sheet 5 has Mn content greater than the invention range, so that the ductility is low, hence offering poor formability. Steel sheet 6 has Ti content less than the invention range, so that the tensile strength after quenching is insufficient under the condition 2. Steel sheet 8 has Ti content greater than the invention range, so that the steel sheet has low ductility, hence offering poor formability. Steel sheet 9 has B content less than the invention range, so that the steel sheet has insufficient tensile strength after quenching under the condition 2. Steel sheet 10 has B content greater than the invention range, so that the steel sheet has low ductility, hence offering poor formability. Steel sheet 14 has an average carbide diameter exceeding 2 μm, so that the steel sheet has insufficient tensile strength at low temperature and short time heat treatment under the condition 3. TABLE 1 Chemical composition (mass %) Steel C Si Mn P S sol. Al N Ti B Cr Mo (48/32)S + (48/14)N 2[(48/32)S + (48/14)N] Remarks 1 0.035 0.03 2.0 0.015 0.008 0.045 0.025 0.025 0.0010 <0.01 <0.01 0.021 0.041 Comparative example 2 0.065 0.03 1.9 0.015 0.008 0.045 0.025 0.025 0.0010 <0.01 <0.01 0.021 0.041 Inventive example 3 0.125 0.03 1.6 0.015 0.008 0.045 0.025 0.025 0.0010 <0.01 <0.01 0.021 0.041 Comparative example 4 0.085 0.03 1.5 0.015 0.008 0.045 0.025 0.025 0.0010 <0.01 <0.01 0.021 0.041 Comparative example 5 0.065 0.03 2.7 0.015 0.008 0.045 0.025 0.025 0.0010 <0.01 <0.01 0.021 0.041 Comparative example 6 0.065 0.03 1.8 0.015 0.008 0.045 0.025 0.012 0.0010 <0.01 <0.01 0.021 0.041 Comparative example 7 0.075 0.03 1.7 0.015 0.008 0.045 0.025 0.031 0.0010 <0.01 <0.01 0.021 0.041 Inventive example 8 0.065 0.03 2.1 0.015 0.008 0.045 0.025 0.050 0.0010 <0.01 <0.01 0.021 0.041 Comparative example 9 0.073 0.03 1.8 0.015 0.008 0.045 0.025 0.025 0.0002 <0.01 <0.01 0.021 0.041 Comparative example 10 0.063 0.03 2.0 0.015 0.008 0.045 0.025 0.025 0.0050 <0.01 <0.01 0.021 0.041 Comparative example 11 0.065 0.03 1.7 0.015 0.008 0.045 0.025 0.022 0.0010 0.50 <0.01 0.021 0.041 Inventive example 12 0.065 0.03 1.8 0.015 0.008 0.045 0.025 0.027 0.0010 <0.01 0.30 0.021 0.041 Inventive example 13 0.065 0.03 2.0 0.015 0.008 0.045 0.025 0.025 0.0010 <0.01 <0.01 0.021 0.041 Inventive example 14 0.073 0.03 1.9 0.015 0.008 0.045 0.025 0.025 0.0010 <0.01 <0.01 0.021 0.041 Comparative example Underline: Outside of Present Invention Range

TABLE 2 Delayed fracture Average Mechanical properties (hydrogen Steel carbide YP TS El Tensile strength after quenching (MPa) embrittlement) sheet Steel diameter (μm) (MPa) (MPa) (%) Condition 1 Condition 2 Condition 3 time Remark 1 1 0.5 301 429 34.9 965 936 946 30 days or longer Comparative example 2 2 0.8 309 442 33.9 1295 1256 1269 30 days or longer Inventive example 3 3 3.5 314 448 33.5 1432 1389 960  3 days Comparative example 4 4 0.9 299 427 35.1 1288 940 1262 30 days or longer Comparative example 5 5 0.8 349 498 28.0 1525 1521 1494 30 days or longer Comparative example 6 6 0.7 305 435 34.5 1266 960 1241 30 days or longer Comparative example 7 7 0.5 305 435 34.5 1291 1253 1266 30 days or longer Inventive example 8 8 0.8 340 485 29.0 1352 1312 1325 30 days or longer Comparative example 9 9 0.7 308 441 34.0 1239 955 1214 30 days or longer Comparative example 10 10 0.7 322 460 29.5 1320 1280 1294 30 days or longer Comparative example 11 11 0.7 300 428 35.0 1358 1317 1331 30 days or longer Inventive example 12 12 0.8 305 435 34.5 1320 1280 1294 30 days or longer Inventive example 13 13 1.5 314 449 33.4 1324 1284 1234 30 days or longer Inventive example 14 14 3.1 313 447 33.5 1338 1298 975 30 days or longer Comparative example Underline: Outside of Present Invention Range

EXAMPLE 2

By using the steels 2 and 7 shown in Table 1, steel sheets A to G were manufactured under manufacturing conditions shown in Table 3. For these steel sheets, quenching under the conditions similar to those in EXAMPLE 1 was conducted, and measurements similar thereto were conducted. The results are shown in Table 4.

Any of steel sheets A, D, E, and F has high ductility (El) leading to excellent formability, tensile strength of 1200 MPa or higher after quenching independently of the conditions, and 30 days or longer delayed fracture time leading to excellent hydrogen embrittlement resistance.

In comparison, steel sheets B and C (comparative examples) each have low ductility, hence offering poor formability. This is because the steel sheet B received rapid cooling not only after hot rolling but also after continuous annealing, and the steel sheet C received low temperature coiling after hot rolling. TABLE 3 Hot rolling conditions Annealing conditions Heating Finishing Coiling Cold Soaking Cooling Steel temperature temperature Cooling rate temperature reduction temperature Holding rate sheet Steel (° C.) (° C.) (° C./s) (° C.) rate (%) (° C.) time (° C./s) Remarks A 2 1230 880 20 600 — — — — Inventive example B 2 1230 870 35 600 — — — — Comparative example C 2 1230 860 20 450 — — — — Comparative example D 7 1230 870 20 600 60 700 2 minutes 15 Inventive example E 7 1230 870 20 600 60 830 2 minutes 20 Inventive example F 7 1230 870 20 600 60 690 2 hours  0.01 Inventive example G 7 1230 870 20 600 60 720 2 minutes 35 Comparative example Underline: Outside of Present Invention Range

TABLE 4 Delayed fracture Mechanical properties Tensile strength after (hydrogen Steel YP TS El quenching (MPa) embrittlement) sheet (MPa) (MPa) (%) Condition 1 Condition 2 Condition 3 time Remark A 309 442 33.9 1295 1256 1269 30 days or longer Inventive example B 357 510 29.4 1320 1280 1294 30 days or longer Comparative example C 387 553 27.1 1273 1235 1248 30 days or longer Comparative example D 302 432 34.7 1280 1242 1254 30 days or longer Inventive example E 295 421 35.6 1260 1521 1235 30 days or longer Inventive example F 279 399 37.6 1265 1227 1240 30 days or longer Inventive example G 352 503 29.8 1291 1253 1266 30 days or longer Comparative example 

1. A steel sheet for heat treatment consisting essentially of, by mass %, 0.05-0.09% C, below 1% Si, 1.6-2.4% Mn, 0.02% P or less, 0.02% S or less, 0.01-0.1% sol.Al, 0.005% N or less, 0.0003-0.003% B, Ti satisfying formula (1), and the rest of Fe: (48/32)S+(48/14)N≦Ti≦2[(48/32)S+(48/14)N]  (1) (the content of each element shown by the element symbol in formula (1) is represented by mass %), wherein the average diameter of iron carbides precipitating in steel is 2 μm or smaller.
 2. The steel sheet for heat treatment according to claim 1, further comprising at least one element selected from 0.1-2% Cr and 0.1-2% Mo.
 3. A method for manufacturing a steel sheet for heat treatment comprising the steps of: hot rolling a steel slab having composition as defined in claim 1 or 2 into a steel sheet; cooling the hot rolled steel sheet at an average cooling rate of 30° C./s or less; and coiling the cooled steel sheet at a coiling temperature of 500° C. or above.
 4. A method for manufacturing a steel sheet for heat treatment comprising the steps of: hot rolling a steel slab containing composition defined in claim 1 or 2 into sa steel sheet; cold rolling the hot rolled steel sheet; and annealing the cold rolled steel sheet for recrystallization, wherein the annealed cold rolled steel sheet is cooled at an average cooling rate of 30° C./s or less to 400° C. after annealing. 